A magnetic bubble memory operative in the "field access" mode is characterized by a pattern of magnetically soft elements, such as permalloy, formed in a plane and coupled to the bubble material. These elements are responsive to a magnetic drive field reorienting, typically by rotation, in the plane of the bubble material. The magnetic drive field is generated by a pair of tuned circuits driven in quadrature from an external clock as is well known. Any drift in the inductance or capacitance of the tuned circuits causes the resonant frequencies of the circuits to vary resulting in changes in current and thus in the phases of the drive field components. These changes, in turn, cause a reduction in the operating margins of the turn, cause a reduction in the operating margins of the memory.
In a practical memory arrangement, each tuned circuit is connected to ground via a transistor arrangement triggered by refresh pulses provided at a clock frequency. When the frequency of the tuned circuits varies, the phase of each circuit varies even more with respect to that of the clock signal due to the Q(&gt;10) of the circuit. A problem thus exists with respect to maintaining the phase relationship between the external clock signal and the currents of the tuned circuits driven thereby when the resonant frequencies of the tuned circuits are different from the clock frequency.